Voluntary muscular weakness and increased fatigue in patients with spinal cord injuries (SCI) are two of the primary impairments underlying decreased functional independence. The mechanisms underlying these impairments are incompletely understood, however, and few effective treatments are currently available. The primary goals of this proposal are to characterize recruitment and rate coding patterns of human motor units (MUs) in patients with spinal cord injuries during steady-state contractions and to quantify the time-varying rate modulation patterns during linearly-varying isometric contractions. The proposed experiments involve use of fine wire intramuscular electrodes to record - from single MUs of tibialis anterior and triceps surae muscles during constant-force or linearly varying isometric contractions. Torque, surface electromyography, and single MU data will be analyzed and quantified in patients with SCI and compared to control subjects. The primary hypothesis of this proposed study is that descending neuromodulatory input to the segmental motor system is interrupted following SCI. This hypothesis will be assessed by comparing MU behavior from both control subjects and patients with spinal cord injury to motoneuron - behavior recorded from reduced animal preparations following neuromodulation. It is our contention that non-linear motor unit behavior in control subjects is qualitatively similar to motoneuron behavior in reduced preparations following neuromodulation. Absence of non-linear behavior in patients with SCI may suggested absence of neuromodulatory input, and would establish a foundation for pharmacological treatments for weakness following SCI..